How are endosymbionts spreading from one ecological community to another?
Ecological communities are a diverse assemblage of many different species involved in a web of interactions with each other (Agrawal et al., 2007). However, rarely, are such communities isolated from each other. There are certain members which are relatively cosmopolitan and interconnect with members of multiple communities (Stireman & Singer, 2003) leading to a metacommunity-wide distribution (Brown et al., 2020). The distribution of these arthropods can lead to horizontal transfer of their resident endosymbionts to distinct ecological communities. Within the soil arthropod community, we have found one such example which can potentially be a source of horizontal transfer of endosymbionts across many other communities. The macropterous form of the planthopperNilaparvata lugens (morph0111, BOLD ID SAEVG089-20, Table S3) was found from leaf litter sampling. N . lugens is a highly destructive pest of rice across tropical Asia and can also survive on other tropical grass species (Khan, Saxena, & Rueda, 1988). It is known to migrate long distances in search of actively growing rice plants (Riley, Smith, & Reynolds, 2003). The presence of N .lugens is unsurprising as our sampling season (October) coincided with the rice harvesting season in North India. N . lugensis known to be infected with several endosymbionts like Wolbachiaand Arsenophnonus (Qu et al., 2013). In the present study, it was found to be infected with Wolbachia ST-163 from the B supergroup. The same Wolbachia sequence type has also been reported fromN . lugens from Southern China (Zhang, Han, & Hong, 2013). This indicates that such invasive pest species can potentially introduce their resident endosymbionts into many different arthropod communities.
Conversely, the presence of very similar endosymbionts in geographically distinct locations can indicate their spread from one ecological community to other. The Wolbachia B supergroup, ST-41, has been detected from a phorid fly (morph0285) in our dataset. The same ST-41 has been found in calyptrate flies (Stahlhut et al., 2010) as well as from several other lepidopterans (Ilinsky & Kosterin, 2017; Narita et al., 2011; Russell et al., 2009; Salunke et al., 2012). This is not unexpected given the diversity of Wolbachia infections. However, what is unexpected is the location of the hosts with ST-41 ranges from North America, Africa, Russia, South and South-Eastern Asia all the way to Japan. Unfortunately, it is difficult to conjecture about the reasons behind such a huge range, as corroborating community-wide data is lacking.
The above two instances testify to the utility of a MLST based approach to understand Wolbachia diversity and spread across global arthropod communities. Moreover, these cases also highlight the importance of collecting community-wide data to understand the probable chain of transfer of these bacteria. Such data can also illuminate similar connections for the spread of Arsenophonus andCardinium if employed with multi-locus data (Jousselin, Cœur d’Acier, Vanlerberghe‐Masutti, & Duron, 2013; Stouthamer, Kelly, Mann, Schmitz-Esser, & Hunter, 2019).
A major goal of endosymbiont research is to explain the tempo and mode of their spread across arthropod communities across the world. We contend that evaluating endosymbiont diversity within specific ecological communities is the key to understand this spread. Such studies would give us specific examples of bacterial strains that are better at spreading as well as uncover specific ecological roles of arthropod hosts which are more amenable to horizontal transfer of their resident endosymbionts. As data from such studies accumulate higher level patterns will emerge which can then be empirically tested.